NKCC1 Knockdown Decreases Neuron Production through GABAA-Regulated Neural Progenitor Proliferation and Delays Dendrite Development

Signaling through GABAA receptors controls neural progenitor cell (NPC) development in vitro and is altered in schizophrenic and autistic individuals. However, the in vivo function of GABAA signaling on neural stem cell proliferation, and ultimately neurogenesis, remains unknown. To examine GABAA function in vivo, we electroporated plasmids encoding short-hairpin (sh) RNA against the Na-K-2Cl cotransporter NKCC1 (shNKCC1) in NPCs of the neonatal subventricular zone in mice to reduce GABAA-induced depolarization. Reduced GABAA depolarization identified by a loss of GABAA-induced calcium responses in most electroporated NPCs led to a 70% decrease in the number of proliferative Ki67+ NPCs and a 60% reduction in newborn neuron density. Premature loss of GABAA depolarization in newborn neurons resulted in truncated dendritic arborization at the time of synaptic integration. However, by 6 weeks the dendritic tree had partially recovered and displayed a small, albeit significant, decrease in dendritic complexity but not total dendritic length. To further examine GABAA function on NPCs, we treated animals with a GABAA allosteric agonist, pentobarbital. Enhancement of GABAA activity in NPCs increased the number of proliferative NPCs by 60%. Combining shNKCC1 and pentobarbital prevented the shNKCC1 and the pentobarbital effects on NPC proliferation, suggesting that these manipulations affected NPCs through GABAA receptors. Thus, dysregulation in GABAA depolarizing activity delayed dendritic development and reduced NPC proliferation resulting in decreased neuronal density.